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27 August 2009 Improvement in triplet exciton confinement of electrophosphorescent device using fluorinated polymer host
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Host materials for phosphorescence organic light-emitting diodes (OLEDs) are required to have wide energy gap to prevent back energy transfer and confine triplet exciton on guest molecules. Carbazole (Cz) has been widely used as a building block for host materials because of its relatively high energy gap. We found that the energy gap of Cz can be widened by fluorination at specific positions. A characteristic of the energy gap widening by fluorination is its controllability by the number and position of fluorine substituents. We synthesized 2,7-difluorocarbazole (F-Cz) and estimated the energy gap of Cz and F-Cz from absorption spectra to be 3.59 eV and 3.71 eV, respectively. To confirm the wide-gap effect of F-Cz on OLED device, we synthesized a solution-processable polymer host, poly(N-vinyl-2,7-difluorocarbazole) (F-PVK), which has F-Cz as pendant groups, and compared it with poly(N-vinylcarbazole) (PVK). The OLED devices investigated consisted of an ITO/PEDOT:PSS/EML/CsF/Al multilayered structure. The poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) layer was spin-coated onto an indium tin oxide (ITO) coated glass substrate. Subsequently, the emission layer (EML) composed of PVK or F-PVK, 1,3-bis[(4-tertbutylphenyl)-1,3,4-oxidiazolyl]phenylene (OXD-7), and a blue phosphorescent dopant, iridium(III)bis [(4,6-difluorophenyl)-pyridinato-N,C2'] picolinate (FIrpic) was spin-coated, and the CsF and Al layers were vapor-deposited. The OLED device with F-PVK showed 1.8 times higher maximum current efficiency (27 cd/A) than that with PVK (15 cd/A). The improved efficiency of F-PVK device can be rationalized by the enhanced triplet confinement effect of polymer host composed of fluorinated carbazole.
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Isao Takasu, Yukitami Mizuno, Shuichi Uchikoga, Shintaro Enomoto, Tomoaki Sawabe, Akio Amano, Atsushi Wada, Jiro Yoshida, and Tomio Ono "Improvement in triplet exciton confinement of electrophosphorescent device using fluorinated polymer host", Proc. SPIE 7415, Organic Light Emitting Materials and Devices XIII, 74150B (27 August 2009);

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